DO vs. MLSS

Attribute	DO	MLSS
Definition	Dissolved Oxygen	Mixed Liquor Suspended Solids
Measurement	mg/L	mg/L
Importance	Indicator of water quality	Indicator of biological treatment process
Monitoring	Continuous monitoring required	Regular monitoring required

Introduction

Dissolved Oxygen (DO) and Mixed Liquor Suspended Solids (MLSS) are two important parameters in wastewater treatment processes. DO refers to the amount of oxygen dissolved in water, while MLSS measures the concentration of suspended solids in the mixed liquor of an activated sludge system. Both parameters play a crucial role in the efficiency and effectiveness of wastewater treatment plants.

Measurement

DO is typically measured in milligrams per liter (mg/L) using a dissolved oxygen meter. This device measures the concentration of oxygen in water by using a probe that detects the amount of oxygen molecules present. On the other hand, MLSS is measured in grams per liter (g/L) and is determined by filtering a sample of mixed liquor through a pre-weighed filter, drying it, and then weighing it again to calculate the concentration of suspended solids.

Importance

DO is crucial for the survival of aerobic bacteria in the activated sludge process, as they require oxygen to break down organic matter in wastewater. Low levels of DO can lead to a decrease in the efficiency of the treatment process and can result in the formation of odorous compounds. On the other hand, MLSS indicates the biomass concentration in the treatment system, which is essential for the removal of pollutants and the overall performance of the plant.

Control

DO levels can be controlled by aerating the wastewater using blowers or diffusers to increase the oxygen transfer rate. By monitoring and adjusting the aeration rate, operators can maintain optimal DO levels for the biological treatment process. In contrast, MLSS levels are controlled by wasting a portion of the mixed liquor from the system to maintain a desired concentration of biomass. This process, known as sludge wasting, helps prevent the accumulation of excess solids in the treatment system.

Impact on Treatment

Low DO levels can lead to poor treatment performance, as aerobic bacteria may become stressed or die off, reducing the ability of the system to remove pollutants effectively. In contrast, high MLSS concentrations can result in poor settling of solids in the clarifier, leading to carryover of solids in the effluent and reduced treatment efficiency. Therefore, maintaining optimal levels of both DO and MLSS is essential for the proper functioning of a wastewater treatment plant.

Relationship

There is a direct relationship between DO and MLSS in the activated sludge process. Adequate levels of DO are necessary to support the growth and activity of microorganisms in the mixed liquor, which in turn contributes to the increase in MLSS concentration. However, excessive MLSS levels can lead to oxygen depletion in the system, resulting in lower DO levels. Therefore, operators must strike a balance between maintaining sufficient DO for microbial activity and controlling MLSS to prevent operational issues.

Conclusion

In conclusion, both Dissolved Oxygen (DO) and Mixed Liquor Suspended Solids (MLSS) are critical parameters in wastewater treatment processes. While DO is essential for the survival of aerobic bacteria and the breakdown of organic matter, MLSS indicates the biomass concentration in the treatment system. By monitoring and controlling both parameters effectively, operators can ensure the efficient operation of the treatment plant and the removal of pollutants from wastewater.